1. Technical Field of the Invention
The present invention relates to a vehicle discharge lamp lighting circuit in which a DC voltage from a DC power source is increased to a predetermined DC voltage, the predetermined DC voltage is converted into a square-wave-shaped AC voltage, and a starting pulse is superposed on the AC voltage to light a vehicle discharge lamp.
2. Description of the Background Art
Previously, in a vehicle discharge lamp lighting circuit of this type, a DC voltage from a DC power source (a battery on the vehicle) is increased to a predetermined DC voltage, the predetermined DC voltage is converted into a square-wave-shaped AC voltage, and a start pulse is superposed on the AC voltage to light a vehicle discharge lamp. FIG. 3 shows the arrangement of a related vehicle discharge lamp lighting circuit. In FIG. 3, reference numeral 1 designates a vehicle battery; 2, a noise filter; 3, a DC-DC converter (or an up-converter having only a boost function); 4, a DC-AC inverter; 5, a control circuit; and 7, a discharge lamp (bulb).
In the vehicle discharge lamp lighting circuit, the noise filter 2 prevents the flow of noise from the DC-DC converter side to the vehicle battery side. The DC-DC converter 3 increases a battery voltage V.sub.BATT supplied through the noise filter to a predetermined DC voltage, which is supplied to the DC-AC inverter 4. The boost operation of the DC-DC converter 3 is controlled by the control circuit 5. The DC-AC inverter 4 converts the DC voltage outputted by the DC-DC converter into a square-wave-shaped AC voltage, which is supplied to a starter circuit 6. The starter circuit 6 operates to superpose a starting pulse on the AC voltage outputted by the DC-AC inverter 4, to light a discharge lamp 7.
The noise filter 2, the DC-DC converter 3, the DC-AC inverter 4, and the control circuit 5 form a ballast circuit 8. The internal impedances of the ballast circuit 8 and the starter circuit 6 cause losses to occur. Therefore, in order to increase the ballast efficiency, and to decrease the generation of heat, the internal impedances of the ballast circuit 8 and the starter circuit 6 are decreased as much as possible. In addition, in order to decrease the noise (restriking noise) produced by the discharge lamp, the inductance of the starter circuit 6 is decreased as much as possible to increase the switching of the square waves. The restriking noise is caused because the arc discharge is momentarily suspended when, in the DC-AC inverter 4, the bridge switching operation occurs to invert the direction of flow of current in the discharge lamp 7.
In the vehicle discharge lamp lighting circuit, Vz=20V, where Vz is the lowest lamp voltage of the discharge lamp 7. More specifically, Vz is the voltage provided immediately after the discharge lamp is turned on in the case where the discharge lamp is turned on within a certain time interval after it was last turned on. A maximum allowable current IL.sub.max to the discharge lamp 7 is 2.6 A. If a current larger than the maximum allowable current IL.sub.max flows to the discharge lamp 7, the bulb must be damaged. On the other hand, for a jump start in a cold environment, the battery voltage V.sub.BATT of the vehicle battery 1 is sometimes set at 24V by connecting two 12V batteries in series. In this case, the battery voltage V.sub.BATT is higher than the lowest tube voltage Vz of the discharge lamp 7, and the boost operation of the DC-DC convert 3 is stopped. In this case, the internal impedances of the ballast circuit 8 and the starter circuit 6 are decreased as much as possible, and therefore a current over the maximum allowable current IL.sub.max flows in the discharge lamp 7. In other words, since V.sub.BATT is larger than Vz, the exceeding voltage is also applied to the lighting circuit. That is, a current IL, which is calculated by dividing the exceeding voltage by sum of the resistances of the entire lighting circuit, and if the current IL is larger than IL.sub.max the bulb is damaged.